Method for producing fine diameter wire from steel-titanium melts

ABSTRACT

A method is provided for preventing orifice plugging when melt extruding a steel-titanium alloy to form fine diameter wire. This is accomplished by controlling the oxygen potential in the melt above the orifice at a level wherein the activity of titania within the melt is maintained at from 0.3 to unity -- the standard state of unit activity being defined as the melt saturated in titania at the concentrations of titanium and oxygen therein and at the melt temperature.

BACKGROUND OF THE INVENTION

This invention relates to improvements in the method wherein steelalloys are melt extruded to produce fine diameter wire.

Until quite recently, it was not possible to fabricate filamentarystructures from metals or metal alloys by the method of melt extrusion.The limiting factor was that the melt viscosity of these materials is solow as to be practically negligible. In other words, the melts of metalsand metal alloys are essentially inviscid.

The problem presented by an inviscid melt when attempting to extrude itto form filaments is that the surface tension of the filamentary jet, asit issues from the shaping die, is so great in relation to its viscositythat the molten stream breaks up before sufficient heat can betransferred for conversion to the solid state.

This intractable problem has now yielded to a unique solution asdescribed in U.S. Pat. Nos. 3,216,076 and 3,658,979. In accordancetherewith, the nascent molten jet, as it issues from the shaping die, isbrought into contact with a gas capable of instant reaction with the jetsurface. The result is the formation of a thin film which envelopes thejet surface. This thin film has been found to be capable of holding thejet stream together until sufficient heat can be transferred to effectsolidification. For example, fine diameter wire may be formed fromaluminum by extruding the melt at an appropriate velocity into an oxygenmedium. When the hot jet issuing from the extrusion orifice contacts theoxygen-containing atmosphere, a stable film of melt insoluble aluminumoxide forms almost instantaneously about the peripheral surface of thejet. In essence, a sheath is formed which protects the filamentary jetor stream against surface tension break-up until solidification takesplace.

The oxide of aluminum is a solid which is insoluble in the non-oxidizedmolten metal. This, of course makes film formation by contact withoxygen below the orifice possible. However, in the instance of ferrousmetals, as for example steel, the iron oxide is soluble in the liquidmelt. Consequently, a film will not form when a molten jet is extrudedinto an oxidizing atmosphere.

A solution to this problem is provided in the teachings of U.S. Pat. No.3,216,076. As disclosed therein, filamentary structures may be formedfrom metals whose oxides are soluble in the non-oxidized molten metal byalloying them with a minor percentage of a compatible metal whose oxideis insoluble in the non-oxidized molten metal. By compatible metal thereis meant a metal or combination of metals having the ability to form analloy. According to U.S. Pat. No. 3,216,076 metals which may be used forthis purpose include aluminum, magnesium, beryllium, chromium, lanthanumand combinations thereof. The particular metal employed is generallypresent in amounts in excess of 0.5% by weight of the alloy. The upperlimit on the quantity of metal which will produce a stable oxide is onlydetermined by the physical characteristics desired in the ultimatefilamentary product. The metal most commonly alloyed with steel foreffecting film formation when extruding steel melts has been aluminum.

The extension of the capability for producing filaments directly fromthe melt to metals like steel constitutes an important advance in theart. However, commercial scale practice of this potentially attractivemethod for producing steel wire has been inhibited by an inability tocontrol the tendency for the orifice to plug during extrusion. Oxidationreactions occurring in the melt prior to extrusion are largelyresponsible for the partial or complete plugging of the orifice.Contributing to this problem has been a premature oxidation of thesecond metal used to stabilize the molten stream of steel. As has beennoted, aluminum is commonly used for this purpose, and it has not beenfound practical to maintain the melt oxygen content at the very lowlevels required for avoiding a premature precipitation of alumina andcomplexes thereof with the oxides of other metal impurities. Suchprecipitates form solid inclusions in the melt which accumulate in theorifice area and tend to plug it. Likewise, a similar problem existswith other metals heretofore proposed for alloying with steel to providea stabilization capability.

It is therefore a principal object of this invention to provide a meansfor extruding a steel alloy melt to form fine diameter wire in theabsence of orifice plugging caused by insoluble oxide formation abovethe extrusion orifice.

It is another object of this invention to provide a means forcontrolling the oxygen potential of the melt upstream from the extrusionorifice during the extrusion of molten steel to prevent the occurrenceof orifice plugging.

Other objects and advantages will become apparent from a description ofthe invention which follows:

SUMMARY

The above objects are achieved when steel alloy melts are extruded inaccordance with a procedure which includes: (1) employing a melt ofsteel alloyed with titanium, with the titanium being present in anamount of at least 0.2 percent by weight of the alloy; (2) maintaining apressurized gas mixture over the melt consisting of an inert gas and anoxygen containing gas; (3) controlling the oxygen potential of the meltupstream from the extrusion orifice at a level wherein the titaniawithin the melt has an activity of from 0.3 to unity -- such controlbeing effected by maintaining the partial pressure of the oxygencontaining gas at an appropriate predetermined value; (4) extruding themelt as a molten filamentary stream directly into an oxygen-containingmedium of sufficient oxidizing capacity to cause titania to precipitateand form a stabilizing film about the surface of the stream; and (5)cooling the film stabilized stream to the solid state.

DESCRIPTION

As above noted, the method of this invention is directed to theproduction of fine diameter wire from a steel alloy by melt extrusion.For purposes of definition, fine diameter wire may be considered as anywire having a diameter of less than about 35 mils. It is well known, ofcourse, that steel is an alloy of iron and carbon. Generally, the carboncontent will be in the range of from about 0.01 to 4.30 by weight of thealloy in steels intended for use in the production of wire products.

According to this invention, steels of the type just described arealloyed with titanium to provide a film-forming component for the meltextrusion procedure. Generally, the titanium concentration will rangefrom between about 0.2 and 5.0 percent on the total weight of the alloy,although there is no process criticality with respect to the upperlimit. That is, the upper limit may be determined merely on the basis ofthe physical characteristics desired in the ultimate product. However,it does appear desirable that the titanium be present in the alloy in anamount of at least 0.2 percent by weight in order to form a stabilizingfilm of the required strength.

The temperatures employed when extruding the melt are critical only tothe extent that they obviously must be at or above the melting point ofthe alloy. Although not required, it is generally good practice to keepthe temperature 10°-20°C. above the liquidus temperature of the alloyduring extrusion to provide a margin for any heat loss which mightoccur. Likewise, the head pressures employed are critical only to theextent that they must impart a sufficient stream velocity to form anefficient jet in accordance with the parameters as set forth in U.S.Pat. No. 3,658,979.

In the film stabilization of inviscid steel jets according to thisinvention, the viscous film is generated by oxidation of the titaniumadded to the steel expressly for that purpose. This is brought about byextruding the titanium-containing molten jet directly into an oxidizingmedium. Thus, as the jet emerges from the extrusion orifice it isimmediately contacted with an oxidizing atmosphere and a film of titaniais caused to form almost instantaneously.

It has now been found that when carrying out melt extrusion operationsin accordance with the procedure as outlined above, the formation oforifice-plugging inclusions can be greatly reduced by maintaining theactivity of titania in the molten mass above the orifice at valuesbetween 0.3 and unity. The standard state of unit activity for thepurposes of this invention is defined as the melt saturated in titaniaat the concentration of titanium and oxygen therein and at thetemperature of the melt.

The activity of titania within the melt is controlled by means of anoxygen-containing gas which is introduced into the system with an inertgas to provide a positive gas pressure for effecting extrusion. That is,the partial pressure of the oxygen-containing gas in the gas mixtureprovides the mechanism for this control. The appropriate partialpressure for any given run will, of course, depend upon the particulargas employed, the carbon and titanium concentrations within the melt andthe melt temperature. With these parameters being known for anycontemplated operation, those skilled in the art can readily calculatethe particular partial pressure values which are needed to accomplishthe desired result.

Among the oxygen-containing gases which may be employed are carbonmonoxide, carbon dioxide, oxygen and steam with carbon monoxide havingparticular advantages in practice. However, since the purpose of the gasis to function merely as an oxygen donor to the melt chemistry, thechoice of an oxygen-containing gas is essentially without limitation.Any suitable inert gas may be employed as the second component in thepressurized gas mixture. For example, argon and helium are commonlyemployed.

As previously noted, the oxygen content in the melt above the orificeshould be controlled at a level which will insure a titania activity offrom 0.3 to unity. Generally, best results are realized when the oxygenlevel in the melt is at or relatively near saturation with respect totitania and the value of the titania activity is from about 0.9 tounity. The reason for this is that the ease of stabilizingtitanium-containing steel jets as they emerge from the extrusion orificeis determined by the amount of oxygen dissolved in the molten jet.Hence, a titanium-containing steel melt which is saturated orsubstantially saturated with oxygen, vis-a-vis titania, is stabilizedwith greater facility than one which is highly under-saturated inrelation to titania.

As previously noted, film stabilization is brought about by extrudingthe titanium-containing molten jet directly into a gaseous medium havinga sufficient oxidizing capacity for causing titania to precipitate andform a film about the peripheral surface of the jet. Although anoxidizing atmosphere rich in carbon monoxide is generally preferred, anyoxygen-containing gas or gas mixture having sufficient oxygen potentialfor effecting titania formation in the molten stream may be employed. Inaddition to carbon monoxide other suitable examples which may bementioned are carbon dioxide, oxygen, sulfur dioxide and steam. Forpurposes of illustration only, the film stabilization chemistry will bedescribed in terms of a carbon monoxide oxidizing medium. It will beunderstood that other oxygen-containing gases could likewise beemployed. The reactions which occur may be set forth as follows:

1. the absorption of gaseous carbon monoxide (CO.sub.(g)) by the liquidjet to give dissolved carbon (C) and oxygen (O).

    2co.sub.(g) ⃡ 2C + 2O

followed by;

2. the reaction of titanium in the liquid steel with dissolved oxygen toform a solid titania (TiO₂(s)) film

2O + Ti ⃡ TiO₂(s) + 2C

the overall reaction is, thus, the sum of reactions (1) and (2).

3. 2CO.sub.(g) + Ti ⃡ TiO₂(s) + 2C

For the stabilizing film of solid titania to form, it is necessary thatthe solubility limit of oxygen on the steel jet surface be exceeded withrespect to titania. This is brought about by exceeding the equilibriumpartial pressure of carbon monoxide in the oxidizing atmosphere intowhich the steel jet is extruded. The total carbon monoxide pressurerequired for stabilization may be defined as follows:

    P.sub.CO *** = P.sub.CO * + P.sub.CO **

where

P_(CO) *** is the total CO partial pressure required for stabilization,

P_(CO) * is the equilibrium partial pressure, and

P_(CO) ** is the driving force required to form a sufficient strongstabilizing film within the required time limit.

It is seen from the above discussion that orifice plugging is avoidedand a proper stabilization of the extruded jet is achieved by an abilityto control the oxygen content within the steel-titanium melt at thedesired level both above and below the extrusion orifice. That is,before the steel passes through the orifice the activity of titania inthe steel melt is controlled to a value of between 0.3 to unity, withfrom 0.9 to unity generally preferred. As soon as the melt exits fromthe orifice as a filamentary jet, the oxygen level is increased and afilm of precipitated titania is thereby formed before varicose breakupof the molten jet can take place.

As a final step in the production of fine diameter steel wire inaccordance with this invention, the film stabilized molten stream or jetis passed into a cooling medium to effect solidification. It isdesirable to utilize a gas with good thermal conductivity for thispurpose. That is, gases such as helium, hydrogen, carbon dioxide,nitrogen or mixtures thereof may be suitably employed with hydrogen andhelium or mixtures of hydrogen and nitrogen being of particularpreference.

For a description of a representative type apparatus which may beemployed for producing fine diameter wire in accordance with thisinvention, attention is directed to the drawing wherein the singleFIGURE depicts a schematic, partially sectionalized, vertical view of aninduction heated extrusion apparatus.

As shown there, such apparatus is comprised of a crucible 2 having abase plate 3, the crucible and base plate being supported on pedestal 4and enclosed within an insulating cylinder 5 and a susceptor 6 employedin conjunction with induction heating coils 7. The unit is pressurizedby gases brought into the head 9 through conduit 8. Sealing rings 10serve to maintain the pressure within the enclosure by preventingleakage past the base plate. The molten metal 1 is forced throughorifice 11 in orifice plate 12 by the gaseous head pressure and emergesfrom orifice 11 as a cylindrical molten jet 13. The nascent jet passesthrough an oxygen-containing gaseous atmosphere contained within cavity14 provided by the pedestal 4. The oxygen-containing gas is brought intocavity 14 via conduit 15.

It is to be understood that the just-described extrusion apparatus ismerely a schematic representation of a typical assembly which may beemployed in the practice of the present invention. Many designvariations are possible and will readily occur to those skilled in theart. For example, all or part of the pressurizing gas mixture could beintroduced into the system by providing a means for bubbling the gasesup through the melt as an alternative or supplementary means to theintroduction above the melt surface as shown in the drawing. Theimportant consideration is that the oxygen-containing gas be provided tothe system at the proper partial pressure. Moreover, a resistance-heatedassembly could be substituted for the illustration induction-heatedunit. The following illustrative example will serve to further amplifythe invention.

EXAMPLE

A steel alloy made from electrolytic iron alloyed with 0.4 percent byweight of carbon and 1.0 percent by weight of electrolytic titanium wasmelted at a temperature of 1550°C., and the temperature was thereafterdecreased to 1540°C. and held at this level. Under a 9.0 psig headpressure provided by a mixture of argon and carbon monoxide gases, themelt was ejected through a 10 mil orifice and thence into a mixture ofcarbon monoxide and helium. During streaming, the partial pressure ofthe carbon monoxide above the melt was maintained at approximately 0.2atmospheres (the equilibrium value for the oxygen-titanium-carbonreaction within the melt). As the molten stream exited from the orifice,this equilibrium value was caused to be exceeded by the partial pressureof carbon monoxide in the gaseous atmosphere (i.e. a mixture of carbonmonoxide and helium) immediately below the orifice. As a consequence,titania precipitated and formed an enveloping film about the peripheryof the extruded stream. The film-stabilized stream then passed through agas cooled tube where it solidified in the form of a fine diameter steelwire. An orifice plugging or erosion problem was not encountered duringthe extrusion.

While there has been described what presently are considered to be thepreferred embodiments of this invention, it will be apparent to thoseskilled in the art that various changes and modifications may be madewithout departing from the invention. It is to be understood, therefore,that the invention is limited only by a proper construction of thelanguage in the claims which follow.

I claim:
 1. In a method for producing fine diameter wire from the meltof a steel alloy wherein said melt is extruded through an orifice as acontinuous molten stream and into an oxygen-containing atmosphere wherea stabilizing film is caused to form about the surface of the stream topreserve its continuity until solidified, the improvement whichcomprises:a. heating to the melt an alloy comprised of steel and atleast 0.2 percent by weight of titanium; b. maintaining a pressurizedgas mixture over the melt consisting of an inert gas and anoxygen-containing gas; c. controlling the oxygen potential of the meltby means of said oxygen-containing gas at a level wherein the titaniaformed in the melt by oxidization of said titanium has an activity offrom about 0.3 to unity; d. causing said melt to extrude through anorifice as a continuous molten stream and into an oxygen-containinggaseous atmosphere having the capacity for increasing the oxygenpotential of said stream to a level wherein said titania is caused toprecipitate and form a stabilizing film about the periphery of saidstream; e. cooling said film-stabilized molten stream to the solidstate.
 2. The method of claim 1, wherein said steel-titanium meltcontains from about 0.01 to 4.3 percent by weight of carbon and fromabout 0.2 to 5.0 percent by weight of titanium.
 3. The method of claim1, wherein said gas mixture over the melt consists of an inert gas and agas selected from the group consisting of carbon monoxide, carbondioxide, oxygen and steam.
 4. The method of claim 3, wherein said inertgas is argon.
 5. The method of claim 1, wherein said melt is extruded asa molten stream into a gaseous atmosphere selected from the groupconsisting of carbon monoxide, carbon dioxide, oxygen, sulfur dioxideand steam.
 6. The method of claim 1, wherein the oxygen containing gasin the gas mixture over the melt is carbon monoxide and the melt isextruded as a molten stream into a carbon monoxide containingatmosphere.
 7. The method of claim 1, wherein the oxygen potential ofthe melt is controlled to where the activity of titania is from 0.9 tounity.
 8. The method of claim 1, wherein hydrogen or helium are employedas a cooling gas to cool said film-stabilized molten stream to the solidstate.